Prosthetic elements, like joint-replacement surgery in larger synovial joints like the hip, knee or the shoulder, can be anchored to the bone either by bone cement, or by cementless fixation like press fit, or anchorage by fibrous and bone ongrowth, or ingrowth in pores. As for now, fixation with bone cement has given the best long-term results both in the elderly as well in younger patients. Consequently, more cementless elements than cemented ones have been removed and subsequently the need for adequate replacement surgery is essential.
For successful replacement surgery it is deemed necessary to preserve as much bone stock as possible. Thus, ideally, a cementless prosthetic element should permit suitable, rigid anchorage to living tissue, like bone, and in addition, be designed so that it can be retrieved without waste of the living tissue stock needed for successful replacement surgery. The object of the present invention is to fulfill these requirements.
Cementless fixation with prosthetic elements having a plurality of indented or raised portions, provided bone ongrowth, may give good fixation to the bone like the femur. Consequently, such elements may be quite difficult to retrieve. However, such indented or raised portions cannot hinder micro-motion at the element/tissue interface. As the micro-motions causes the patients pain on walking, there is a present need for eliminating or at least minimizing these micro-motions. Moreover, such instability leads to undesirable bone remodeling. Such designs have therefore given poor results in patients.
Replacement of the prosthetic element is necessary in many occasions. Statistics have shown that such revisions have been made in a number of 1 to 6 in relation to primary operations. This illustrates the demand of finding good solutions for removing such prosthetic elements.
Precision press fit of prosthetic elements, like a hip stem, may give good anchorage to bone provided bone tissue ongrowth to a rough surface. The more rough the surface, the better the anchorage. But the more rough the surface, the more difficult the retrieval. Thus, the retrieval of a rough-surfaced element can be very cumbersome. And loss of essential bone stock is inevitable.
Several means have been proposed for the anchorage of the prosthetic element to bone by bone ingrowth. One of these methods involves the application to a portion of the surface of the implant, such as a hip implant, of a coating of sintered balls.
The aim of this method is to provide surface porosity to encourage bone ingrowth. Such a method has several disadvantages like adversely affecting the properties of the material, difficulties to control the surface geometry of the ball coated implant, and surface balls can migrate from the implant causing severe third body wear in the artificial joint proper. Further, the removal of a totally ball coated hip or shoulder joint stem can be disastrous; even if a major part of the bone is temporarily removed, there is the risk of breaking the bone in several pieces.
Mesh pads on a portion of the implant have also been used for anchorage in inventions like U.S. Pat. No. 4,406,023 of Sep. 2. 1983, William H. Harris inventor, and U.S. Pat. No. 4,536,894 of Aug. 27. 1985, Jorge O. Galante et al. inventors. Like sintered balls, these mesh pads give good anchorage. Although a collar obstructing the access to the stem below is provided only medially in the above inventions, removing such implants can be very difficult. And loss of essential bone stock is inevitable.
Austin More designed his cementless hemi prosthetic implant for fractured neck of the femur with a smooth surface facilitating both insertion and possible retrieval. To obtain proximal load transfer a collar was provided. And there are two quite large openings in the stem for bone block anchorage. But such anchorage cannot hinder undesirable micro-motion at the bone/tissue interface. Moreover, to retrieve such prosthesis the anchoring bone blocks have to be removed either by a saw or using a chisel. A collar naturally obstructs the access to the stem below. Thus, to remove the anchorage a quite large access may be needed, the larger the more loss of bone stock.
To obtain more bony anchorage than the above Austin More design patent U.S. Pat. No. 5,330,536 Femur portion of a hip, Jul. 19, 1994, Karl H. Täger and Hans E. Harder inventors, adds multiple Austin More-type-holes to a hollow prosthesis. Naturally, the more anchoring holes the better the fixation, but the more difficult the retrieval if deemed necessary. Provided the interior of the stem is packed with bone tissue, anchorage by bone ingrowth through these quite large holes may be induced. The hollow space, extending from the upper to lower part of the prosthesis, is conical tapering distally so to aid compressing, from above, to the spongy tissue filled therein. The object of this invention is to “design an effective configuration for the stem of a hip joint prosthesis by which the charging with spongy material is facilitated”. Although some of this spongy material might be removed from the hollow space of the prostheses, this procedure alone does not loosen the prosthesis sufficiently to be able to remove the prosthesis from the femur. The removal of such prosthesis involves cutting bone material loose from prosthesis by an extensive approach to the outside of the prosthesis. This operation has features similar to the method used for the totally ball coated stem described above, and results in a relatively extensive loss of bone stock.
For proximal load transfer a collar may be desirable. U.S. Pat. No. 4,623,353 discloses a stem-type femoral prosthesis including a collar provided with access slots for resectioning means, Nov. 18. 1986, Fredrick F. Buechel and Michael J. Pappas inventors, have therefore provided one access slot on each side of a collar. This access slot gives access to the proximal part of the stem, but using a saw or chisel onto the outside of a prosthetic element will inevitably lead to loss of bone stock. Moreover, the slots above give access to the uppermost part of the stem only.
The invention EP 0078 888 Gerader, blattartiger Schaft für eine Gelenkendoprothese, Jul. 24. 1982, Maurice E. Müller inventor, was designed without a collar obstructing the access to the quite large Austin-More-type-holes in the stem for anchoring bone blocks. To reduce the problems of retrieval somewhat this invention also includes a shallow, longitudinal gutter partly guiding the chisel along the implant when hammering off the anchorage. However, as outlined above, such large bone block anchorage cannot hinder undesirable micro-mobility at the implant/bone interface. Second, on retrieval, the chisel (or saw), inevitable destroys bone stock. The more distal the cutting, the more bone stock loss. If not, the femur will crack.
U.S. Pat. No. 6,187,012 B1 a prosthetic element removal apparatus and methods, Feb. 13, 2001, Michael A. Masini inventor, incorporates a “guide means direct a cutting tool into the interface between a prosthesis and surrounding bone to bring about a more controlled separation thereof for revision or other purposes.” This guide means might be on the outside of the prosthesis, or located in the vicinity of the outside of the prosthesis as externally open, semicircular, parallel gutters with an opening in the outside surface of the prosthesis. These guides must be parallel and straight, or the external cutting tool will be stuck. So these guides can only be used in the upper part of a curved prosthetic element. And as outlined above, using a cutting tool into the interface between prosthesis and the surrounding bone will inevitably lead to loss of bone stock, or the bone will crack.
EP 181586 concerns a prosthetic element comprising a prosthetic main part provided with projecting ribs distributed around the circumference of the prosthetic main part. The ribs extend in the longitudinal direction along an upper portion of the prosthetic main part. Several through holes are formed in the ribs for the ingrowth of bone tissue. A possible removal of the prosthetic element would cause considerable loss of bone. The space between the longitudinal ribs may provide some guidance for the cutting tool, but the shape of these spaces does not provide protection so as to minimize the loss of bone stock. Further, each outer edge of the ribs is cogged and the distal portion of the prosthetic element has plural structures to ensure the fixation of the prosthetic element. As the overall design indicates the intention of the prosthetic element is to provide a prosthesis which ensures a reliable fixation to the femur, while the easy removal of the prosthesis has been no object of the invention of EP 181566.
Several means have been proposed for anchoring cementless prosthetic elements, and, as outlined above, some few for facilitating their retrieval. But the better the anchorage, the more difficult the retrieval. And retrieval of the above prosthetic designs inevitable lead to loss of bone stock, or the bone will break in two or more pieces, or both. Such alternatives are undesirable. In contrast, the present invention seeks to provide implants avoiding such serious complications on retrieval.